1. Field of the Invention
This invention relates to an elongated, free standing support, and more particularly, but not exclusively, to a free standing load support suitable for a mine prop.
2. Description of the Related Art
In the field of free standing supports, both yielding and non-yielding supports are useful as an alternative to other supports such as timber crib supports. In particular, free standing supports have been widely used in coal mining, hard rock mining, and deep tabular mines. In addition, free standing supports are used in numerous non-mining applications. Examples of such applications include support for construction sites, basement support, and for emergency conditions, such as shoring up damaged structures during and after fires.
Ideally, a free standing support should be able to accept loads of 20-200 tons. This is especially important in mining operations, as well as non-mining operations. It is desirable to provide a range of installations heights with a single support unit, and a good area of coverage of the mine roof for such a support would be at least 64 in2.
With respect to yielding supports, yield is generally needed in a support because around an excavation, the rock is subjected to natural and mining induced stresses. These stresses will result in the rock tending to fracture into slabs usually sub-parallel to the walls of the tunnel. The depth and severity of these fractures are site and rock type specific, depending on factors such as the magnitude and direction of the stress of the rock, the amount of fracturing caused by blasting operations, and geological features/weaknesses such as bedding planes, joints, dykes and slips. The process of excavating the rock using explosives also causes fracturing and dilation. In general, yielding supports are used in situations which need a permanent support.
With respect to non-yielding supports, non-yield is generally needed in situations where it is desired to re-use the props and/or the area is not expected to undergo much movement. Since yielding props tend to experience a controlled damage, non-yielding props are better adapted for removal and reinstallation. Non-yielding props are therefore useful as temporary supports, especially in situations where little or no movement is expected.
Conventionally, hydraulic type units that can yield using a pressure relief valve can be used as temporary supports, but they tend to be costly and need an external power source to activate them.
Timber based props yield by making a collapsing area on the top or bottom of the pole. These types of yielding supports are more cost effective, but have several disadvantages, including: time consuming set up since they must be cut to size; difficult to transport and install because they are bulky and heavy; pose a fire risk; and their performance is variable and deteriorates over time as the timber loses moisture and becomes brittle.
Steel yielding props are known, but suffer from installation problems because it is necessary to insert wedges, tighten clamps, etc. and this is time consuming.
Conventionally, non-yielding supports take at least one minute to engage. They also tend to be heavy if designed for a high load.
Also, both yielding and non-yielding props usually require at least two persons to install, thus, resulting in high costs and manpower requirements.
U.S. Pat. No. 1,491,229 describes a shore for construction work which has a temporary support/locking device. The locking device is spring activated with a pocket which tapers upward and bearings urged by a spring. The bearings are retained in the pockets by means of the plungers and springs. Tools are required to adjust the locking device, which is inconvenient and time consuming.
U.S. Pat. No. 3,991,964 is directed to a telescoping prop for building construction. A housing is mounted at the top of the lower tube for locking the upper and lower tubes relative to each other. Bearings are disposed in a tapered area of the housing, wherein the taper has a step structure for holding the bearings in place. A locking device is required in conjunction with the bearings, which is inconvenient and complicated.
U.S. Pat. No. 6,299,113 is directed to a telescopic prop for furniture use, such as for adjusting the heights of chairs, tables, etc. Frictional resistance is provided to hinder the relative movements of the inner and outer cylinders. The mechanism merely produces a braking force, and is not a load mechanism capable of supporting high loads.